Influence of processing parameters, crystallography and chemistry of defects on the microstructure and texture evolution in grain-oriented electrical steels

Yilmaz, Ceren; Zaefferer, Stefan (Thesis advisor); Raabe, Dierk (Thesis advisor); Schneider, Jochen M. (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022


Grain oriented electrical steels (GOES), based on the composition Fe-3 wt. %Si, are soft magnetic materials that are widely used as core material in electrical transformers. In order to achieve the required magnetic properties for minimized magnetic losses, the microstructure of GOES should consist of very large grains (up to the range of few centimetres) with sharply oriented in Goss orientation ({011}<100>). Production of GOES is achieved by a complex industrial process chain, where abnormal growth of Goss oriented grains takes place during box annealing. Despite a century of research, the underlying mechanism that leads to the abnormal growth of Goss grains is not well understood. This limits the ability to tune the process parameters, which determine the final microstructure and texture properties, and therefore, the final magnetic properties of GOES. Understanding how Goss texture forms and how the process parameters influence the final microstructure and texture is of great interest for both academia and industry.In the present thesis, among the process parameters the influence of ageing treatment during cold rolling, which improves the final magnetic properties of GOES, is investigated by means of large-area electron backscatter diffraction (EBSD). Initially identical hot rolled sheets were cold rolled by following two different routes, namely with ageing and without ageing. Subsequently, primary recrystallization annealing was performed in a dilatometer furnace with the aim of tracking the evolution of microstructure and texture. A new approach for defining a benchmark, indicating the statistical relevance of the EBSD-based texture measurements was developed. It is based on the deviation from the perfect symmetry. Accordingly, the large area EBSD measurements were confirmed to provide enough statistics for a representative texture investigation for the heavily cold rolled and the recrystallized GOES sheets. In addition, to determine true local misorientations from the ontained EBSD results in the not-fully recrystallized samples, a new methodology was developed, where noise-corrected kernel average misorientations were calculated based on a second order polynomial relationship between the kernel average misorientations and kernel size. The obtained results revealed that ageing during cold rolling significantly influences the deformation behaviour and consequently, the recovery and recrystallization kinetics in α-fiber regions. The α-fiber regions in the cold rolled and aged material exhibit a higher shear deformation, caused by the decoration of dislocations with dissolved carbon atoms. The interaction of solutes with dislocations is promoted by ageing. The dislocation pinning leads to a hindered recovery during annealing, which finally results in a higher remaining driving force for recrystallization and more nucleation sites in the otherwise sluggishly recrystallizing α-fiber ({hkl}<110>) grains. As a result a smaller recrystallized grain size is observed in the material that was cold rolled with ageing. The effect of ageing on the improved final magnetic properties of fully processed GOES sheet was, therefore, attributed to the smaller primary recrystallized grain size, which in turn, provides a larger driving force for Goss grain growth during the subsequent secondary recrystallization step, due to a higher total area of grain boundaries.The final part of the thesis focuses on the formation of Goss texture in grain-oriented electrical steels via abnormal grain growth. Fe-3.1 wt. % Si grain-oriented electrical steels were processed in the industry plant of Thyssenkrupp Electrical Steel GmbH according to the standard production. Correlative controlled-electron channelling contrast imaging (cECCI) and atom probe tomography (APT) experiments were performed for structural and elemental analysis of dislocations, respectively. Solute enrichment at grain boundaries were analyzed using APT for the samples taken from various steps of the production chain of GOES. The results indicated that the abnormally grown Goss-oriented grains contain very low angle grain boundaries, i.e. periodically arranged, <111> type mixed dislocations. These sub-grain dislocations are locally enriched with solutes such as Sn, Cu, C, and more importantly, with Al, N and Mn, which belong to the composition of inhibitor particles which assist the abnormal growth of Goss-oriented grains, by suppressing normal grain growth of the matrix grains.Experimental observations regarding the segregation of Al on subgrain dislocations were supported by molecular statics (MS) simulations. Based on these findings, a new hypothesis called "Dislocation-assisted particle dissolution" is proposed to explain the abnormal grain growth of Goss-oriented grains. According to this theory, subgrain dislocations originate most probably from the shear bands and microbands that form during cold rolling. These dislocations then survive primary recrystallization, and finally assist the abnormal growth of Goss grains by aiding the dissolution of inhibitors and by diffusing and absorbing the solutes from the boundaries. As a result, boundaries of Goss-oriented grains become more mobile compared to the boundaries of matrix grains.


  • Division of Materials Science and Engineering [520000]
  • Chair of Materials Physics and Institute for Physical Metallurgy and Materials Physics [523110]